Wacky Offset Values

[Frederick Farah]

I was under the impression that the Offset_sec value in the TransitView API is the time in seconds since the vehicle's location was last updated. To check this, I logged data for one specific vehicle on the 42 route once per second, and this is an example of what I got over a short period of time:

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 38 Timestamp: 16:43:29.850382

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 13 Timestamp: 16:43:31.405827

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 21 Timestamp: 16:43:32.603631

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 20 Timestamp: 16:43:33.652024

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 21 Timestamp: 16:43:34.703553

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 20 Timestamp: 16:43:35.753127

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 13 Timestamp: 16:43:36.801074

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 26 Timestamp: 16:43:38.334871

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 21 Timestamp: 16:43:41.201037

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 26 Timestamp: 16:43:42.284423

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 21 Timestamp: 16:43:43.333258

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 20 Timestamp: 16:43:44.398771

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 20 Timestamp: 16:43:45.448736

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 33 Timestamp: 16:43:46.500658

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 26 Timestamp: 16:43:47.555616

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 33 Timestamp: 16:43:48.883786

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 33 Timestamp: 16:43:49.937248

lat: 39.949432 lng: -75.15233600000001 Offset_sec: 39 Timestamp: 16:43:51.342079

I would expect offset_sec to go up by approximately 1 every second, and then reset around 0 once the location is updated. Instead, the numbers increase and decrease randomly in a way that isn't consistent with how seconds even work, if you look at the timestamps to the right. 

Is my interpretation of Offset_sec wrong, or is something else weird going on?

[Greg Apessos]

Hello,

   Sorry for the delay.  Your interpretation of the offset_sec is correct.  However, because of the infrastructure in place, the strange jump in offset is a result of the path the request took and where/when the caching took place.  (The buses currently have a 30 second polling cycle and the cache is set to expire after 15 seconds.)  In retrospect, using a timestamp might have been better choice instead of an offset value. 

   We will take a look at this tomorrow and see if we can clean that up.

-Greg